This invention relates to fluid-driven actuator devices It is particularly but not exclusively intended for use in a system in which one or more packages is to be ejected from a container.
The conversion of fluid pressure and motion to linear displacement by means of a piston or bellows is well known. A piston-based device requires a rigid cylinder within which the piston is caused to move by fluid pressure, and the length of the cylinder must correspond to the displacement or stroke of the piston. If the piston is used to eject a package from a container at a substantial velocity, either the stroke is short, the cylinder is short and the ejection shock relatively high; or the stroke is made longer and the shock is reduced, but appreciable packaging space is lost because a larger rigid cylinder is required. If a device essentially comprising a bellows is used as an ejector, unless the displacement path of the bellows during its extension is defined by some rigid guide such as a surrounding tube, the body of the bellows will tend to distort away from the ideal because the system cannot remain symmetrical and stable.
Another form of actuator may be based on a flexible sac, which may be stored in a deflated state requiring relatively little packaging space, and expanded by inflation to provide the actuation movement. Known devices perform in an unpredictable manner during inflation of the sac. Furthermore, if rapid inflation is required the configuration of the sac is further complicated by transient phenomena associated with fluid flow into the sac and the nature of any external load. When fully inflated, however, the sac will adopt a shape determined either by the walls of the sac or by container into which it has been expanded, or by some combination of the two.